Between the individual vertebral bodies of the spinal column, the intravertebral disks are located, better known as simply "disks". The disks comprise an outer ring of connective tissue and cartilage and an inner gelatinous core, and they serve as elastic buffers between the vertebral bodies. The inner gelatinous core can shift, however, or prolapse through cracks in the outer ring. This disorder is known as prolapse or herniation of the disks. In severe cases, nucleotomy or diskotomy is necessary; that is, the disk has to be removed. After that the vertebral bodies have to be fused; that is, the spacing between them must be maintained by suitable provisions; otherwise, they would slip into the gap that was previously filled by the disk. This would cause compressions in this region of the spinal column; the spinal cord would be compressed and stretched in the vicinity of the affected vertebral bodies. The consequence would be pain and neurological defects.
Fusion of the vertebral bodies can be done for example by means of intravertebral implants.
One known intravertebral implant is known by the term PLIF (for posterolateral interbody fusion), in which cagelike hollow bodies (intersomatic cages) are inserted between the vertebral bodies. To that end, in the region previously occupied by the disk, a space into which the hollow body is driven is created by roughening the adjacent vertebral bodies or removing their periosteum and the cortical substance. A disadvantage of this is that in this method additional stabilization is always necessary, for instance internal fixation by means of four pedicle screws connected to two rods. Even then, the hollow bodies can slip because they grow into the implant only gradually. This is due to the fact that the cortical substance, which is jointly responsible for bone grown, is involved. Especially, the implants can be forced away toward the rear, until they have grown firmly into place.
Another possibility known from the prior art is to implant screwable hollow cylinders for lumbar interbody fusion. These implants are hollow cylinders with a male thread. This means that a thread has to be cut into the vertebral body adjacent to the disk space. The hollow cylinders are then screwed into that thread. In this method, it is true that the implants can no longer slip, but the method is extremely complex, which also entails the risk of complications.
The necessity of injuring the periosteum and the cortical substance of the vertebral bodies is also felt to be a disadvantage, because these structures are hard and solid and normally have a stabilizing effect. The bone growth and thus the fusion are slowed down by the injury to the cortical substance.
One disadvantage common to both these known intravertebral implants is their fixed dimensions. However, the spacing to be maintained between the affected vertebral bodies is different for each patient. Hence numerous versions with different dimensions are needed. Nevertheless, it cannot be avoided that the intravertebral implants are not seated optimally, for instance being seated too firmly or too loosely.
The object of the invention is therefore to furnish an adjustable intravertebral implant of the above type, with which the above-described disadvantages are avoided and with which in the simplest possible way, fast and durable fusion of vertebral bodies is made possible.
This object is attained in that the hollow body can be expanded on at least two opposed sides and locked in the expanded position by an adjusting element that can be introduced into the hollow body, and the expansion is done on the end at which the adjusting element can be introduced.
The object of the invention is also to furnish a fast, simple method for expanding an intravertebral implant.
The method according to the invention contemplates the following method steps:
(a) prewidening the hollow body; PA1 (b) expanding the hollow body.
The object of the invention is also to furnish instruments for implanting an intravertebral implant.
Instruments according to the invention are defined by the characteristics recited in claims 17, 22, 23 and 24.
The intravertebral implant of the invention has fixation elements which secure it on the vertebral bodies so that it will not change position. These elements anchor the intravertebral implant to the vertebral bodies. The expansion can be done to a variable extent, so that variable dimensions are available. That is, the diameter of the intravertebral implant and hence the spacing of the vertebral bodies from one another are adjustable. Thus the correct size and in particular the correct diameter for every patient is always available. Moreover, the intravertebral implant is always precisely correctly seated, namely not too firmly and not too loosely. The play is approximately 1 to 2 mm. Substantially fewer intravertebral implants of different dimensions are needed for the varying vertebral body spacings of the patients. The widening elements are also used to retract the fixation elements into the "insertion position", so that the hollow body can be driven in without hindrance between the vertebral bodies. Only after that is the expansion done, which thus brings about the fixation of the fixation elements on the vertebral bodies.
The expansion of the intravertebral implant can be done in various ways. One possibility is to provide spreader tongues, which are cutout from the hollow body and can be bent outward and which have fixation elements on their outer surface. Another possibility is for the hollow body itself to have a slot of a certain width, which thus divides it into two halves that can be pressed together or expanded. In that case, the play is not defined by the diameter of the hollow body itself.
It is especially advantageous for the intravertebral implant of the present invention to be designed with a somewhat crowned or bellied form. Thus it is especially well adapted to the contours of the space, formed by the surfaces of the vertebral bodies, that was previously filled by the disk.
The expansion of the intravertebral implant according to the present invention is preferably done in two steps. Since the expandable regions are first contracted, it is prewidened in a first step, until the fixation elements touch the vertebral bodies. In a second step, it is then widened to the final extent and thus fixed on the vertebral body. The fixation elements have advantageously have a relief the inside of which can be engaged by a retrieval instrument, so that they can optionally be pulled back in again after the expansion.
The instruments necessary for the implantation include an implant holder, with which the intravertebral implant of the present invention can be guided easily and securely until its final fixation. This implant holder is preferably hollow and can receive other instruments within it as well, such as a distractor for the prewidening or an insertion instrument for introducing the adjusting element. Hence it is assured that the location and seating of the intravertebral implant can be monitored at all times during the insertion process.
The subject of the present invention is also an instrument for measuring the spacing between two vertebral bodies. It is thus possible to find out what size of implant is needed or how much the implant has to be expanded.